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/* $NetBSD: n_tanh.c,v 1.1 1995/10/10 23:37:08 ragge Exp $ */
/*
* Copyright (c) 1985, 1993
* The Regents of the University of California. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. Neither the name of the University nor the names of its contributors
* may be used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
#ifndef lint
static char sccsid[] = "@(#)tanh.c 8.1 (Berkeley) 6/4/93";
#endif /* not lint */
#include "math.h"
/* TANH(X)
* RETURN THE HYPERBOLIC TANGENT OF X
* DOUBLE PRECISION (VAX D FORMAT 56 BITS, IEEE DOUBLE 53 BITS)
* CODED IN C BY K.C. NG, 1/8/85;
* REVISED BY K.C. NG on 2/8/85, 2/11/85, 3/7/85, 3/24/85.
*
* Required system supported functions :
* copysign(x,y)
* finite(x)
*
* Required kernel function:
* expm1(x) ...exp(x)-1
*
* Method :
* 1. reduce x to non-negative by tanh(-x) = - tanh(x).
* 2.
* 0 < x <= 1.e-10 : tanh(x) := x
* -expm1(-2x)
* 1.e-10 < x <= 1 : tanh(x) := --------------
* expm1(-2x) + 2
* 2
* 1 <= x <= 22.0 : tanh(x) := 1 - ---------------
* expm1(2x) + 2
* 22.0 < x <= INF : tanh(x) := 1.
*
* Note: 22 was chosen so that fl(1.0+2/(expm1(2*22)+2)) == 1.
*
* Special cases:
* tanh(NaN) is NaN;
* only tanh(0)=0 is exact for finite argument.
*
* Accuracy:
* tanh(x) returns the exact hyperbolic tangent of x nealy rounded.
* In a test run with 1,024,000 random arguments on a VAX, the maximum
* observed error was 2.22 ulps (units in the last place).
*/
double tanh(x)
double x;
{
static double one=1.0, two=2.0, small = 1.0e-10, big = 1.0e10;
double t, sign;
if (isnan(x))
return (x);
sign=copysign(one,x);
x=copysign(x,one);
if(x < 22.0)
if( x > one )
return(copysign(one-two/(expm1(x+x)+two),sign));
else if ( x > small )
{t= -expm1(-(x+x)); return(copysign(t/(two-t),sign));}
else /* raise the INEXACT flag for non-zero x */
{big+x; return(copysign(x,sign));}
else if(finite(x))
return (sign+1.0E-37); /* raise the INEXACT flag */
else
return(sign); /* x is +- INF */
}
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